Abstract
Early school start times (SST) force adolescents to wake earlier than biologically preferred, creating chronic sleep debt. While evidence supports delaying SSTs, few controlled studies have used objective measures to assess their impact. We conducted a controlled intervention in a French boarding school to evaluate the effects of a one-hour delay in SST on sleep (N = 50) and cognitive functioning (N = 73) in early adolescents (age 12.8 years [11.7–14.2], 66% girls). After a baseline period with 8 a.m. starts (T0), four classes were randomized: half remained at the early schedule (Control-SST), half switched to 9 a.m. (Delayed-SST). Sleep measured by actigraphy, cognitive performance and mental health were assessed at baseline (T0) and 6 months later (T1). Between T0 and T1, total sleep time decreased in the Control-SST group, whereas the Delayed-SST group showed a modest, non-significant increase. This resulted at T1 in a 26-min between-group difference (Cohen’s d = 0.93, p = 0.007). Sleep onset time did not differ between groups. Sleepiness decreased in Delayed-SST but increased in Control-SST (d = -0.52, p = 0.042). Inhibitory control improved in Delayed-SST compared to Control-SST (d = -0.79, p = 0.001), with trends toward better sustained attention (d = -0.40, p = 0.051). Delaying SST by one hour appears to mitigate the progressive reduction in sleep duration commonly observed during adolescence, and benefits both cognitive functioning and sleepiness outcomes in early adolescence.
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Acknowledgements
We would like to acknowledge to all participants, as well as to the teachers and the school head, whose time, effort, and support in coordinating schedules and facilitating the assessment were instrumental in making this study possible. We would also like to acknowledge the French National Education Scientific Council (Conseil scientifique de l’Éducation nationale) for the financial support. This project was made possible thanks to the support of the IDEE program funded by the French State via the National Research Agency (ANR) as part of the Investissements d’avenir/France 2030 program, reference number: ANR-21-ESRE-0034.
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We would also like to acknowledge the National Education Scientific Council (Conseil scientifique de l’Éducation nationale) for the financial support and the IDEE program (ANR-21-ESRE-0034) the administrative, technical and statistical support.
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Reynaud, E., Malevergne, L., Grellet, A. et al. A one-hour delayed school start improves sleep, sleepiness and inhibitory control in early adolescents in a randomized controlled trial. Sci Rep (2026). https://doi.org/10.1038/s41598-026-50892-6
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DOI: https://doi.org/10.1038/s41598-026-50892-6
